Resin-cemented optical element, mold therefor, fabrication process thereof, and optical article

ABSTRACT

NK1717 The present invention provides a resin-cemented optical element comprising a base member  10  and a resin layer  11  formed on the surface of the base member; the resin layer  11  being in a thickness of 300 μm or smaller at least at some part of a peripheral portion (i.e., a region within 1 mm from the peripheral edge face  17  of the resin layer  11,  or a region outside an effective-diameter region), and being in a thickness 12 of 850 μm or larger at a position which is thickest in the resin layer; a mold therefore; a manufacturing method thereof; and an optical article having this optical element.

[0001] This application is based on Japanese Patent Application Nos.2000-365992 and 2001-231933 filed in Japan, the contents of which areincorporated hereinto by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a resin-cemented optical element, amold used for producing the element, an optical article (or device)having the element, and fabrication process of the element.

[0004] 2. Description of the Related Art

[0005] At present, optical elements are used in various fields.Depending on the purpose for which they are used, it is difficult tomaterialize required optical characteristics and so forth in some casesin respect of conventional spherical lenses. Accordingly, asphericlenses are attracting notice. “Aspheric lens” is a generic term forlenses the curvature of which is kept continuously different over theregion extending from the lens center toward the periphery. The use ofaspheric lenses at some part of optical systems enables considerablereduction of the number of lenses necessary for the correction ofaberrations, compared with a case where the optical system is comprisedof only spheric lenses. This enables downsizing and weight reduction ofthe optical system. Also, the use of aspheric lenses enables high-gradecorrection of aberrations which is difficult for spherical lenses, andhence can bring about an improvement in image quality.

[0006] Aspheric lenses having such superior characteristics have notnecessarily come into wide use. The greatest reason therefor can be saidto be a difficulty in working. Conventional aspheric lenses have only beable to be produced by precisely polishing base members made of glass,and have involved the problem of a high processing cost.

[0007] In recent years, however, resin-cemented optical elements thatmaterialize aspherical shapes by the aid of resin layers which can bemade into any desired shapes with much greater ease than the precisepolishing of glass have been put into practical use, so that asphericlenses have rapidly come into wide use.

[0008] The resin-cemented optical element is an element in which a resinlayer has been cemented to the surface of a base member made of glass orthe like. This resin-cemented optical element is produced by a processsuch as a composite-type aspherical-surface molding process, in which,using a mold (such as a metal mold), a resin composition (inclusive of aresin precursor composition) is poured into a space between a basemember and the mold, followed by curing to form on the base membersurface a resin layer having any desired shape. In the presentspecification, a lens produced by this composite-type aspherical-surfacemolding process may be called a PAG(plastic adhesion glass) lens.

[0009] In the case when the resin-cemented optical element is thusproduced by the composite-type aspherical-surface molding process, thebase member may break when the resin cured on the base member isreleased from the mold. This phenomenon is remarkable especially whenthe resin layer has a large thickness. Accordingly, it has beenimpossible in practice to produce any PAG lens having a thick resinlayer of 850 μm or larger in maximum layer thickness.

[0010] This phenomenon is considered to be caused by the adhesion of theresin layer to the mold. Usually, the resin is released from the mold bymeans of an ejector (ejection member) in such a way that a force actingin the direction where the former is released from the latter is appliedto the base member at its part standing uncovered to the periphery ofthe element. Here, in the event that the resin layer remains withoutbeing released, in the state it has been kept adhered to the mold untilthe amount of deformation of the base member exceeds a tolerance limit,the base member breaks because of the distortion due to a deformationhaving exceeded the tolerance limit.

SUMMARY OF THE INVENTION

[0011] Accordingly, an object of the present invention is to provide aresin-cemented optical element having a thick resin layer, withoutcausing any break of the base member, and to provide a mold used forproducing the element and an optical article having the element.

[0012] To achieve the above object, the present invention provides aresin-cemented optical element comprising a base member and a resinlayer formed on the surface of the base member, wherein the resin layeris in a thickness of 300 μm or smaller at least at a part of aperipheral portion (i.e., a region within 1 mm from the peripheral edgeface of the resin layer, or a region outside an effective-diameterregion), and is in a thickness of 850 μm or larger at a position whichis thickest in the resin layer.

[0013] The present invention also provides a mold for forming a resinlayer of a resin-cemented optical element having a base member and aresin layer formed on the surface of the base member, wherein the moldhas, on the outer periphery on the outside of a molding surface, aconcavely curved surface which has a curvature larger than the moldingsurface. It still also provides an optical article having theresin-cemented optical element of the present invention and afabrication process of the element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

[0015]FIG. 1 is a cross-sectional view showing an example of theconstruction of the optical element according to the present invention;

[0016]FIG. 2 is a cross-sectional view of an optical element having astair;

[0017]FIG. 3 is an illustration showing an angle at which a normal ofthe base member surface falls with a resin layer tangent plane;

[0018]FIG. 4 is a cross-sectional view showing an example of theconstruction of the optical element according to the present invention;

[0019]FIGS. 5A and 5B illustrate the steps of producing an opticalelement in Example 1;

[0020]FIG. 6 is a graph showing resin thickness in the optical elementproduced in Example 1;

[0021]FIG. 7 is a cross-sectional view showing a mold used in Example 2;

[0022]FIG. 8 is a cross-sectional view showing a peripheral portion ofmolding surface of the mold used in Example 2; and

[0023]FIGS. 9A to 9C illustrate the steps of producing a mold used inExample 2.

DETAILED DESCRIPTION OF THE INVENTION

[0024] In the resin-cemented optical element of the present invention,as shown in FIG. 1, a resin layer 11 has a thickness of 300 μm orsmaller (preferably 100 μm or smaller) at least at some part of aperipheral portion (i.e., a region within 1 mm from the peripheral edgeface of the resin layer 11, or a region outside the effective-diameterregion), and has a thickness of 850 μm or larger (preferably 1 mm orlarger) as the maximum value of the thickness of the resin layer 11.Also, in order to attain necessary strength, optical characteristics andso forth, the resin layer 11 may preferably be formed usually in athickness of at least 20 μm, without regard to the inside or outside ofthe peripheral portion.

[0025] Incidentally, what is shown in FIG. 1 takes the case of anoptical element whose resin layer molding surface is convex, to which,however, the present invention is by no means limited.

[0026] The resin layer may have the thickness of 300 μm or smaller atits whole peripheral portion, but may be enough as long as it has thethickness of 300 μm or smaller at least at some part of the peripheralportion. This is because the resin in the vicinity where a force forpeeling is applied at the time of mold release may have layer thicknessin this value. In the present invention, the resin present within 1 mmin periphery from the resin layer edge face closest to the part to whicha force for mold release is to be applied (i.e., the part against whichan ejector is to be pressed) may be in the thickness of 300 μm orsmaller.

[0027] Here, the peripheral portion is meant to be a region within 1 mmfrom the peripheral edge face of the resin layer 11, or a region outsidethe effective-diameter region. A region inside the effective-diameterregion is meant to be a region through which light rays used in opticaldesigning are transmitted, thus the region outside theeffective-diameter region is meant to be a region except for thisregion. In general, the resin thickness of an element is strictlydetermined in accordance with the required optical characteristics.However, as long as it is in the region outside the effective-diameterregion, it does not affect any optical characteristics of the element.Hence, the layer thickness can appropriately be selected.

[0028] In the optical element of the present invention, the resin layermay preferably have layer thickness which becomes gradually smallertoward the periphery, at least at some part of the peripheral portion.Making the resin layer have such a thickness that does not form anystair so as not to have any abrupt change in thickness is preferredbecause not only molds can be produced with easy but also any defectscan be prevented that may occur because the resin can not turn aroundwhen a resin composition is poured into the mold.

[0029] According to the present invention, even a resin-cemented opticalelement having characteristic features a to k as shown below, havingbeen considered impossible in elements having the resin layer of 850 μmor larger in maximum layer thickness, can be produced in a good yieldwithout causing any break at the time of mold release.

[0030] a. As shown in FIG. 1, the resin layer 11 has a maximum layerthickness 12 which is at least four times a minimum layer thickness 13.

[0031] b. The resin layer 11 has a total mass of 700 mg or larger.

[0032] c. The resin layer 11 has an external diameter of 34 mm orlarger.

[0033] d. The base member 10 has a thickness of 10 mm or larger asmaximum value.

[0034] e. The base member 10 has a thickness of 1 mm or smaller asminimum value.

[0035] f. The base member 10 has an external diameter of 35 mm orlarger.

[0036] g. As shown in FIG. 2, the base member 10 has a resin layer 11molding surface which is a concave surface, and the base member 10 hasalong its periphery a stair 16 which protrudes in the peripheraldirection (e.g., an attachment part for fastening the base member to alens barrel). Incidentally, hatching is omitted in FIG. 2 in order tomake the illustration easy to view.

[0037] h. As shown in FIG. 3, an angle at which a normal 21 of theinterface 20 between the base member 10 and the resin layer 11 fallswith a tangent plane 22 on the outside of the resin layer is 80° orsmaller as minimum value.

[0038] i. As shown in FIG. 4, the base member 10 has a resin layer 11molding surface which is a concave surface, and the resin layer 11 hasan external diameter 14 which is at least 1.2 times a curvature radius31 of the concave surface.

[0039] j. As shown in FIG. 4, the base member 10 has a resin layer 11molding surface which is a concave surface, and the resin layer 11molding surface has a curvature radius 31 of 24 mm or smaller.

[0040] k. The base member has a resin layer molding surface which is aconvex surface, and the resin layer has an external diameter which is atleast 1.2 times a curvature radius of the convex surface.

[0041] According to the present invention, a resin-cemented opticalelement having a resin layer with a large maximum layer thickness can beobtained in a good yield.

[0042] There are no particular limitations on the base member used inthe optical element of the present invention. Sol-gel glass, inorganicglass and organic glass may be used. Usually used are transparentmaterials having a refractive index of nd=1.4 to 2.0 and νd=20 to 100 inapproximation. However, an opaque material or a semitransparent materialmay be used as the base member where the resin is not cured by exposureor, even when cured by exposure the resin can be exposed to light on theside of the mold.

[0043] Components constituting the inorganic glass may include, e.g.,SiO₂, B₂O₃, P₂O₅, Na₂O, K₂O, CaO, BaO, MgO, ZnO, PbO, MnO, Al₂O₃ andFe₂O₃. The organic glass may include poly(methyl methacrylate),polystyrene, poly(vinyl chloride), polyester, celluloid, and cellulosederivatives.

[0044] There are no particular limitations on the resin that constitutesthe resin layer in the present invention, and any of photosensitiveresins, thermosetting resins and thermoplastic resins may appropriatelybe selected as long as they can be molded by means of a mold. Thethermosetting resins, which are suited for the present invention, mayinclude, e.g., epoxy resins, urethane resins, thiourethane resins,unsaturated polyester resins, diallyl phthalate resins, and diethyleneglycol bisallyl carbonate known under a trade name CR-39. Also, thethermoplastic resins may include poly(methyl methacrylate), polystyreneand polycarbonate. Photosensitive acrylic resins and photosensitivemethacrylic resins are also preferable for the present invention.

[0045] A resin composition used in the optical element of the presentinvention may preferably have a viscosity before polymerization curing,of 50,000 cP or lower at room temperature. If it has a viscosity higherthan 50,000 cP, a poor operability may result and besides some failuredue to inclusion of bubbles may greatly occur.

[0046] The resin composition used in the present invention mayappropriately optionally contain, in addition to the resin (or aprecursor thereof), a polymerizing agent (curing agent), apolymerization initiator, a release agent, an anti-scratching agent andso forth.

[0047] The polymerizing agent and the polymerization initiator mayappropriately be selected depending on the type of and curing conditionsfor the resin to be used, required film properties and so forth. As thereleasing agent, usable are, e.g., neutralizable or non-neutralizablephosphate alcohols. The anti-scratching agent has the effect ofsmoothing the surfaces of cured products to improve resistance toscratching, and keeping any faults from occurring. This anti-scratchingagent may include silicon oxides such as tetramethoxysilane,tetraethoxysilane, γ-methacryloxypropyltrimethoxysilane,γ-glycidyloxypropyltrimethoxysilane, and acrylate or methacrylate havingan Si—O bond at some part of the backbone chain.

[0048] The resin-cemented optical element of the present invention mayinclude, e.g., lenses, prisms and diffraction gratings. the presentinvention can bring about superior effects especially when applied toaspheric lenses. The present invention may also be applied to asphericmirrors.

[0049] In particular, the optical element of the present invention issuited to optical articles (or devices) such as still cameras especiallyrequired to be made small-size and/or light-weight, such as analog stillcameras and digital still cameras, video cameras, and interchangeablelens systems for these cameras, as well as spectacles, telescopes,binoculars, microscopes and optical disk/magneto-optic disk readingpickup lens systems. Accordingly, the present invention also providesthese optical articles having the optical element of the presentinvention.

[0050] The resin layer of the resin-cemented optical element of thepresent invention can be formed by means of a molding tool the moldingsurface of which has an inverted shape of the resin shape describedabove. According to the molding of the resin layer of the resin-cementedoptical element of the present invention, the thickness of a resin wellformed on the outside of the resin layer outer edge at the time ofmolding can be made much smaller. Hence, in particular, it is preferableto use at a peripheral portion outside a molding surface (hereinafteroften simply “molding surface peripheral portion”) a mold having aconcavely curved surface which has a larger curvature than the moldingsurface. Accordingly, the present invention provides a mold for moldingthe resin layer of the resin-cemented optical element; the mold havingat its peripheral portion outside a molding surface a concavely curvedsurface which has a larger curvature than the molding surface. In such amold, the resin layer surface (its base member side being regarded asthe back) may be either of a concave surface and a convex surface. Itmay be used also for the molding of a lens having both the concavesurface and the convex surface. It is effective especially when used forthe molding of a concave lens (a lens the resin layer surface of whichhas a concave shape).

[0051] The concavely curved surface at the molding surface peripheralportion (hereinafter often simply “concavely curved surface”) maypreferably be at least 0.1 mm outside the effective-diameter region, andmore preferably be at least 0.2 mm outside the effective-diameterregion. Also, the edge of the curved surface (the position at which thecurved surface begins as viewed on the inside of the effective-diameterregion, i.e., the position at which the curvature changes from thecurvature of the molding surface) may preferably be not distant by 0.5mm or more from the effective-diameter region. The concavely curvedsurface may preferably be so hollowed inward that a cross sectionembracing an axis corresponding to the optical axis of the resin layerto be molded forms an inverted arc. Its curvature may appropriately bedetermined as long as it is larger than that of the molding surface, andmay usually be 0.6 to 1.5 mm in radius. The distance between the basemember of the element to be molded and the outer edge of the mold maypreferably be so set as to be 1 mm or shorter.

[0052] The mold of the present invention may be produced by cutting orgrinding, depending on materials. For example, where the mold isproduced from a cuttable material such as electroless nickel plating(nickel formed by electroless plating), motion transfer type cutting maybe performed by the use of a cutting tool having a cutting surface witha small curvature. This enables formation of the concavely curvedsurface at the molding surface peripheral portion of the mold.

[0053] However, such a cuttable material, though moldable with ease,often has a short lifetime because it tends to be scratched. Also, thismethod requires replacement of the cutting tool in the course ofcutting, resulting in a high cost and besides tending to produce adifference in height at the surface formed.

[0054] Accordingly, in order to achieve mass production of opticalelements, it is preferable to produce the mold with use of a hardmaterial such as single-crystal silicon, SiC, CVD(chemical vapordeposition)-SiC, WC, SKD or hardened steel. These materials, however,can not be shaped by cutting, and must be shaped by grinding. Thus, inthe method of motion transfer type cutting, the cutting object and thecutting tool may interfere with each other, and hence any concavelycurved surface having a large curvature (i.e., having a small curvatureradius) can not be formed.

[0055] Accordingly, in the case when the mold is produced by grinding, aform grinding wheel having a grinding surface in an inverted shape of atleast part of the molding surface of the mold to be produced may be usedso that the shape of the grinding surface can be transferred. Thus, amold having the concavely curved surface having a large curvature can beobtained. According to the method in which the shape of the grindingsurface is transferred using the form grinding wheel in this way, even aconcavely curved surface having a small curvature radius (e.g., acurvature radius of 3 mm or smaller) can be formed at a low cost and asintended.

[0056] This form grinding wheel can be produced by, e.g., cutting acuttable material such as brass to prepare a form grinding wheel baseoriginally, and bonding abrasive grains to its grinding surface. Here,as the abrasive grains, it is preferable to use hard abrasive grainssuch as particles of single-crystal or polycrystalline diamond or CBN(cubic boron nitride). Also, the abrasive grains may be bonded byplating with a nickel alloy or the like.

THE PREFERRED EMBODIMENTS

[0057] In the following Examples, the resin layer is irradiated by light(ultraviolet rays) on the side of the base member, and a mold made ofmetal is used as the mold. The present invention, however, is by nomeans limited to these. For example, a transparent material such asglass may also be used as the mold. In the case when such alight-transmitting material is used as the mold, the resin compositioncan be cured by irradiation on the mold side, and hence the base memberneed not be transparent.

EXAMPLE 1

[0058] 80 parts by weight of dimethacrylate represented by the followingstructural formula (1), having a weight-average molecular weight of 800,19.5 parts by weight of urethane-modified hexamethacrylate representedby the following structural formula (2) and 0.5 part by weight of anacetophenone type photoinitiator were mixed to prepare a photosensitiveresin composition,

[0059] As shown in FIG. 5, on the resin layer molding surface (concavesurface) of a glass base member (BK7) of 40 mm in external diameter(diameter), 1 mm in center thickness, 10 mm in maximum thickness and 18mm in resin layer molding surface curvature, having been subjected tosilane coupling treatment to improve adhesion to resin, the above resincomposition, 51, was dropped. The glass base member 10, with its upsidedown, was pressed against a convex surface of an aspheric metal mold 52to press and spread the resin composition 51 into the desired shape.Thereafter, on the base member side, the resin composition wasirradiated by ultraviolet rays 53 for 5 minutes by means of ahigh-pressure mercury lamp at an illumination of 10 mW/cm² to effectcuring to form a resin layer 11 having thickness distribution shown inFIG. 6.

[0060] Subsequently, the glass base member 10 was pushed with an ejectorat the former's peripheral portion 54 to release the resin layer 11 fromthe metal mold 52 to obtain a PAG lens. Here, the resin layer was formedin an external diameter of 38 mm, a maximum resin thickness of 850 μm, aresin thickness outside the effective-diameter region (within 1 mm fromthe peripheral edge of the resin layer), of 300 μm or smaller, and aresin quantity of 700 mg. The amount of deformation of glass at the timeof mold release was 20 μm.

[0061] The resin layer of the PAG lens obtained in the present Examplehas a large aspherical shape in a maximum thickness of 850 μm and aminimum thickness of 100 μm. Even though the resin layer was molded insuch a large aspherical shape, the desired aspherical shape was exactlytransferred, and a PAG lens having a precise aspherical surface wasobtainable without any break of the base member at the time of moldrelease. Ten PAG lenses were produced in the same manner as the above.As the result, any break of the base member did not occur at all in allthe lenses.

EXAMPLE 2

[0062] In the present Example, a PAG lens was molded using a metal moldhaving the concavely curved surface at the molding surface peripheralportion. A cross section of an aspherical surface metal mold 70, cutalong a plane embracing an axis 71 (in the present Example, the axis ofrotation) corresponding to the optical axis of the lens to be molded, isshown in FIG. 7. An enlarged view of its peripheral portion 72 is shownin FIG. 8 in a state held at the time of molding.

[0063] The metal mold 70 used in the present Example has, as shown inFIGS. 7 and 8, a concavely curved surface 73 at the molding surfaceperipheral portion. This concavely curved surface 73 is so formed thatthe cross section embracing an axis 71 corresponding to the optical axisof the resin layer to be molded forms an inverted arc having a curvatureradius of 1 mm. The position 83 at which the curved surface begins asviewed on the side of the effective-diameter region is kept at 0.3 mmoutside the effective-diameter region (diameter: 33.4 mm) of the lens tobe molded.

[0064] According to the mold 70 of the present Example, the concavelycurved surface 73 is provided at the molding surface peripheral portion.Hence, a resin well 81 can be made much thinner than a resin well 82formed at the time of molding when the metal mold 52 of Example 1 isused.

[0065] The metal mold 70 of the present Example was produced in thefollowing way. First, brass was cut to originally prepare a formgrinding wheel base 90 shown in FIG. 9A, and nickel alloy plating wasapplied to its grinding surface by the use of a plating solution mixedwith abrasive grains. Thus, as shown in FIG. 9B, a form grinding wheel92 having a plating layer 91 having abrasive grains on its surface wasobtained. The grinding surface of this form grinding wheel 92 has aninverted shape of the molding surface of the metal mold 70 to be ground.More specifically, a convexly curved surface 93 is provided at the innerperiphery of the grinding surface.

[0066] Subsequently, as shown in FIG. 9C, the grinding wheel 92 wasrotated and a grinding fluid 96 was fed to the grinding surface, duringwhich the surface of the mold 70 on its molding surface side was pressedagainst the grinding surface of the grinding wheel 92 to transfer theshape of the grinding wheel 92 to the surface of the mold 70. Thus, themold 70 shown in FIGS. 7 and 8 was obtained, having the concavely curvedsurface 73 at the peripheral portion on the outside of the moldingsurface.

[0067] PAG lenses were produced in the same manner as in Example 1except that the aspherical mold 70 produced as described above was usedin place of the aspherical mold 52. As the result, any break of the basemember did absolutely not occur in all the lenses.

COMPARATIVE EXAMPLE 1

[0068] Ten PAG lenses were produced in the same manner as in Example 1except that the resin layer outside the effective-diameter region was ina layer thickness of 800 μm. The amount of deformation of glass at thetime of mold release was 80 μm on the average. Of the ten base members,five were seen to break.

[0069] While we have shown and described several embodiments inaccordance with our invention, it should be understood that disclosedembodiments are susceptible of changes and modifications withoutdeparting from the scope of the invention. Therefore, we do not intendto be bound by the details shown and described herein but intend tocover all such changes and modifications a fall within the ambit of theappended claims.

We claim:
 1. A resin-cemented optical element comprising a base memberand a resin layer formed on a surface of the base member, wherein: saidresin layer is in a thickness of 300 μm or smaller at least at a part ofa region within 1 mm from the peripheral edge face of the resin layer;and said resin layer is in a thickness of 850 μm or larger at a positionwhich is thickest in said resin layer.
 2. The resin-cemented opticalelement comprising a base member and a resin layer formed on a surfaceof the base member, wherein: said resin layer is in a thickness of 300μm or smaller at least at a part of a region outside aneffective-diameter region; and said resin layer is in a thickness of 850μm or larger at a position which is thickest in that layer.
 3. Theresin-cemented optical element according to claim 2, wherein: at leastat a part of the region outside an effective-diameter region, said resinlayer has a thickness which becomes gradually smaller toward theperiphery.
 4. A mold for molding a resin layer of a resin-cementedoptical element having a base member and said resin layer formed on thesurface of the base member, wherein said mold has, on the outerperiphery on the outside of a molding surface, a concavely curvedsurface which has a curvature larger than the molding surface.
 5. Anoptical article comprising the resin-cemented optical element accordingto claim
 1. 6. An optical article comprising the resin-cemented opticalelement according to claim
 2. 7. An optical article comprising theresin-cemented optical element according to claim
 3. 8. A fabricationprocess for a resin-cemented optical element having a base member and aresin layer formed on the surface of the base member, comprising, a stepof molding said resin layer with a mold having, on the outer peripheryon the outside of a molding surface, a concavely curved surface whichhas a curvature larger than the molding surface.